Viscous-Elastic Polymer Can Increase Microscale Displacement Efficiency in Cores

Abstract

Abstract When flooding with viscous Polymers, due to the increase in viscosity and decrease in permeability (for polymers that absorb on the rock surface), the mobility ratio compared to water flooding becomes more favorable. Therefore the volumetric sweep efficiency increases and the recovery of the reservoir on a macro scale increases also. Most people think that polymer flooding does not increase the recovery at a micro scale. But numerous results in this work do not support this conclusion. From cores in the lab, the effect of flooding with viscous-elastic polymers on different types of residual oil after water flooding was studied. The different types of residual oil are:oil film on the rock surface;oil in "dead ends";oil in pores throats retained by capillary forces;oil un-swept in micro scale heterogeneous portions of the core. It could be seen that all types of micro-scale residual oil were reduced after flooding with viscous-elastic polymers. Due to the elastic nature of the polymer, the velocity distribution in the pores are quite different from Newtonian Fluids and the polymer could also exert a very strong "pulling effect" on different types of residual oil. The study pointed out that the relationship between Capillary Number and Recovery of cores for Newtonian Fluids does not apply to fluids with elastic properties. In the study, it was seen that the increase in micro scale recovery was related to the elastic properties of the polymer fluids. Different polymer fluids had quite different elastic properties. Some had practically no elasticity. Therefore, when selecting polymers for flooding, its elastic properties must be considered. The difference in incremental recovery can be more than 6% OOIP (original oil in place), which is substantial, it can make a polymer flood successful (both technically and economically) or not. The above conclusions can also be confirmed by pressure coring data and field results of large scale polymer (PAM) flooding in Daqing Oil Field, which is obtaining an incremental oil recovery of more than 13% OOIP. This magnitude of incremental oil recovery can not be explained by just an increase in volumetric sweep efficiency. The above conclusionscan explain why some polymer floods were successful or not;should be considered when selecting polymer floods and,should be able to increase the incremental oil of future polymer floods. The rheology of viscous-elastic fluids in porous media is very much different from fluids with no elasticity. Much work needs to be done in this field. Many conclusion obtained by studies on Newtonian Fluids need to be re-assessed when used on visco-elastic fluids. Introduction The relationship between Capillary Number and Recovery when flooding cores with Newtonian Fluids is well-established[1]. It is known that to obtain substantial increase in recovery at a micro scale in cores, the Capillary Number needs to be increased several thousand times, but the increase when flooding with polymers, compared to with water, usually is less than one hundred. Therefore, most people think that polymer flooding does not increase the recovery at a micro scale. However, in the lab, the recovery from natural and artificial consolidated cores were mostly 5~8% OOIP higher when polymer flooded than water flooded. The same results were obtained by flooding glass-etched cores. The end point (oil saturation when water cut reaches 100%) of relative permeability curves were 6~8% lower for polymer flooding than by water flooding. Pressure cores in polymer flooded portions of the reservoir showed residual oil saturation much lower than could be obtained by water flooding.